The invention relates to a method for accurately measuring a longitudinal contour of a measuring object, in particular suitable for measuring internal or external screw thread contours and similar profiles, wherein use is made of a measuring device comprising clamping members for retaining a measuring object to be measured; a scanning arm which is pivotable about an axis transverse to a longitudinal center line of the clamped measuring object and whose free end comprises at least one scanning needle, extending transverse to the scanning arm, for canning a surface of the measuring object, the scanning arm being mounted on a driving device which, in operation, can move the scanning arm in at least one linear direction of movement parallel to the longitudinal center line of the measuring object, and means being provided for detecting the angular displacement of the scanning arm and the position of the scanning arm in the at least one linear direction of movement; and a processor for processing information regarding the angular displacement and the position of the scanning arm.
A method of the above-described type is known from, for instance, European patent application 0097537, British patent applications 2114746 and 2052065, and German patent application 3023607. A similar manner, in which, however, the scanning arm does not pivot but rotates about its own longitudinal center line, is known from applicants European patent 0589500.
Known devices for using a method of the above-described type are, for instance, the Conturograph from the firm Carl Mahr, the Contracer from the firm Mitutoyo, and the Form Taleysurph from the firm Rank Taylor Hobson.
A drawback of the known technique is that when for instance a screw thread profile or a groove for an O-ring or the like is measured, the scanning needle of the scanning arm is drawn in one direction, parallel to the longitudinal center line of the measuring object, over the surface to be scanned, while the scanning needle may catch behind the flanks of the screw thread or the like. This problem particularly occurs in screw threads or other contours having steep flanks, such as for instance trapezoidal thread according to DIN standard 103, or grooves for, for instance, O-rings or screw thread having straight flanks. If the scanning needle catches behind a flank, this may cause damage to the measuring equipment and/or inaccurate measurements. In the past, it has been proposed to solve this problem by setting the scanner into vibration. However, this too has an adverse effect on the accuracy of the measurement, as well as on the lifetime of the measuring equipment.
It is an object of the invention to overcome the drawbacks outlined and in general to provide an improved, accurate and reliable method and device for measuring a longitudinal contour of a measuring object. To this end, according to the invention, a method of the above-described type is characterized in that a contour measurement is made in such a manner that during the performance of a pivotal movement of the scanning arm, which involves the scanning needle moving along a contour flank, the driving device drives the scanning arm in such a manner that the scanning needle moves along a contour flank in downward direction only, i.e. in the direction of the bottom of a profile groove.